The present invention relates to a gas turbine blade cooling structure, and, more particularly, to a gas turbine blade cooling construction suitable for cooling the blade with an air bled from a compressor.
Thermal efficiencies of a gas turbine can be increased by elevating the temperature of a motive gas or a combustion gas directed to the blade. The inlet gas temperature has been raised by about 300.degree. C. during the last ten years due to progress in cooling techniques and to developments of heat-resistant material. In particular, significant progress in cooling techniques has contributed to the elevation of the inlet gas temperature.
In conventional cooling methods, however, the blade to be cooled has an nonuniform distribution of temperature, that is, the temperature is highest at the central portion of the blade height, and becomes lower away from the central portion. Therefore, the blade has a large temperature differential between its central portion, its tip, and its root. This differential is due to heat transfer carried out between the combustion gas and a dilution gas, a material defining gas passages for the motive gas, etc. As a result, the motive gas becomes higher in temperature in the central portion of the blade height and lower in temperature in the other portions. This temperature distribution is parabolical. The blade is influenced greatly by the parabolical temperature distribution of the motive gas and, thus, it has a large temperature differential. This temperature differential is not desirable because thermal stresses are thereby induced in the blade with the result that fatigue and breakage of the blade take place and a long blade life cannot be expected.
The central portion of the blade is weaker in strength because it is subjected to the highest temperature combustion gas and has relatively large stresses due to centrifugal force occurring during the blade rotation. Therefore, in order to make it stronger, it is necessary effectively to lower the temperature at the central portion of the blade in view of blade design without lowering the thermal efficiencies.
An example of a prior art gas cooled turbine blade is shown in Japanese Patent Publication No. 48-24084. The applicants herein have also filed a copending patent application entitled "Gas Cooled Turbine Blade" filed on July 10, 1979 and given Ser. No. 056,225.